WASHINGTON (AP) — They are the ghosts of the House impeachment hearings.

Vice President Mike Pence. Secretary of State Mike Pompeo. Energy Secretary Rick Perry. Acting White House Chief of Staff Mick Mulvaney. Rudy Giuliani. And perhaps most tantalizingly, the mustachioed John Bolton, President Donald Trump's former national security adviser.

These principals closest to Trump have hovered over the House's impeachment hearings, only the fourth such presidential inquiry in U.S. history. Often mentioned through five days and a dozen witnesses but absent from the room, they sit at the center of a...